Magnetic resonance imaging (MRI) is a medical diagnostic imaging technique used to diagnose many types of injuries and medical conditions. An MRI system includes a main magnet for generating a main magnetic field through an examination region. The main magnet is arranged such that its geometry defines the examination region. The orientation of the main magnet defines whether the MRI system is classified as a horizontal field system or a vertical field system. In a horizontal field system, the static main magnetic field is typically oriented in the head-foot (H-F) direction relative to the prone/supine patient within the system. In a vertical field system, the static magnetic field is typically oriented in an anterior-posterior (A-P) direction relative to the prone/supine patient within the system.
The main magnetic field causes the magnetic moments of a small majority of the various nuclei within the body to be aligned in a parallel or anti-parallel arrangement. The aligned magnetic moments rotate around an equilibrium axis with a frequency that is characteristic for the nuclei to be imaged. An external radiofrequency (RF) field is applied as a pulse by other hardware within the MRI system. The RF pulse perturbs the magnetization from its equilibrium state. Upon termination of the application of the RF pulse, the magnetization relaxes to its initial state. During relaxation, a time varying magnetic moment is introduced which induces a detectable time varying voltage. The time varying voltage can be detected by a receive coil (e.g., an imaging coil).
The receive coil typically comprises one or more coil elements. Each coil element can typically comprise a continuous piece of copper formed in a loop, butterfly, figure-eight, or other continuous geometric shape. Each coil element can include inductive and capacitive elements. The coil elements can be positioned at various locations throughout the receive coil to provide for a desired imaging of the patient. The coil elements operate by resonating and efficiently storing energy at particular frequencies commonly called a Larmor frequency. The particular Larmor frequency at which the coil elements operate can depend upon the particular object being imaged (e.g., a knee, a brain). The design of the receive coil can vary based upon its use in a vertical or horizontal field MRI system.
One or more RF receive coils are typically placed within the vicinity of a patient to facilitate magnetic resonance imaging. During operation of the receive coil, each coil element collects information from the time varying voltage induced by the magnetic moments from the patient's anatomy that is nearest to each coil element. The information collected by each coil element is processed through the electronics associated with the MRI system. The information from each coil element can be on an individual MRI system channel to keep the information from each coil element separate during the imaging process. The information from each MRI system channel can be processed by reconstruction software associated with the MRI system. The reconstruction software can combine the single images from the MRI system channels to create an overall image of a patient's anatomy being imaged.
The shape, configuration, and/or location of coil elements can affect the characteristics of the receive coil, such as sensitivity, signal-to-noise ratio (SNR), and imaging field-of-view (FoV) (e.g., distance between two points on the receive coil's sensitivity profile where the signal drops to 80% of its peak value). Smaller coil elements typically provide higher sensitivity and SNR, but decreased FoV, while larger coil elements provide lower sensitivity and SNR, with an increased FoV. Considering this, receive coils commonly utilize numerous smaller elements positioned over the entirety of the receive coil, rather than very few larger elements that cover the entirety of the receive coil.
When two coil elements of a receive coil having the same resonance frequency are brought into close proximity to one another, the common resonance frequency can split into two separate frequencies (e.g., due to the electromagnetic interaction or coupling between the two coil elements). Two coil elements are considered to be magnetically coupled if each coil element induces a net non-zero magnetic flux linkage in the other coil element. However, two coil elements are considered to be magnetically de-coupled, or isolated, if each coil element induces a net zero magnetic flux linkage in the other (e.g., completely nullifying the magnetic flux linkage between each other). Generally, the closer the coil elements are in proximity with each other, the stronger the resulting magnetic coupling is. When two or more coil elements are in close proximity with each other, the coil element can magnetically couple, thereby causing sensitivity degradation in a receive coil having a maximum sensitivity that is optimized for a particular, relatively narrow, band of frequencies.
Within the art, attempts have been made to isolate coil elements that are in close proximity to each other within an RF receive coil. It is known within the art that coil elements are commonly decoupled from one another by using the geometry and configuration of the elements, by overlapping elements, or by using capacitive or inductive decoupling. Coil element isolation has also been achieved using separate preamplifiers with respective coil elements of a receive coil (e.g., commonly known as preamp decoupling). Conventional isolation techniques are limited by the amount of preamp isolation and/or geometric isolation that can be achieved. Also, these conventional receive coil isolation arrangements often require the receive coil and coil elements to be properly loaded. Proper loading is oftentimes accomplished by placing an appropriate patient anatomy or a phantom load (e.g., simulation of the load commonly associated with a particular patient anatomy) within the receive coil during the receive coil tuning and isolation of the receive coil. Conventional isolation techniques are not desensitized to this receive coil loading of the receive coil. Additionally, conventional isolation techniques do not facilitate optimal tuning of a receive coil that is designed to accept different loads (e.g., a receive coil that is designed to image the knee on certain occasions, and the ankle on other occasions).